1Field of the Invention
The present invention relates to a constant speed drive control unit for adjusting an opening of a throttle valve automatically so as to equalize a running speed of a car with a desired speed from comparing the two.
2. Description of the Prior Art
A constant speed drive control unit operates a throttle so as to adjust a current car running speed to a desired speed from increasing an opening of the throttle valve to acceleration when the car running speed is lower than the desired speed through comparison or from decreasing the throttle valve opening to deceleration when the former is higher, however, particularly at the time of descending where a speed change mechanism is set at high speed stage, the running speed becomes higher than the desired speed, and even from minimizing the throttle valve opening, the running speed does not decrease, and while the throttle valve opening gets gradually small, the running speed does not come up with the desired speed for unsatisfactory engine brake effect.
For prevention, for example, according to the constant speed driving gear disclosed in Japanese Patent Laid-Open No. 2740/1988, when the throttle valve is full close and the running speed is increasing, a speed stage of the speed change mechanism is changed (or shifted down) to a low speed stage.
However, in a car provided with fuel injection controller (or electron-controlled fuel injector, which is called "EFI" hereinafter), a fuel supply to an engine is subjected to an optimal control on throttle valve opening, air flow, engine rotational frequency and others by means of an electronic controller such as microcomputer or the like, and the fuel supply is cut (called "fuel cut" hereinafter) when the engine runs at a predetermined rotational frequency or over with the throttle valve in idling opening (may be called "full close" for example). This is an effective manner of control to cause enhancement of the power of an engine brake and enhancement of a fuel consumption, however, the following disadvantages may result:
That is, if the constant speed drive control unit drives and closes the throttle valve for suppressing acceleration of a car at running on a steep descent, then the valve may be closed full sometimes. In this case, a fuel cut condition is realized and EFI operates accordingly, however, if a car speed is decreased suddenly from having a brake power applied to the car from the engine for the fuel cut, the constant speed drive control unit then drives to open the throttle valve so as to suppress a deceleration of the car, therefore the fuel cut condition comes unrealized this time, and thus EFI removes the fuel cut (feeding the fuel). The car speed is increased again by resupply of the fuel, thus repeating what has been mentioned above to surging of the car.